Turbomachines such as axial turbines or compressors have an outer casing that houses an inner casing; the inner casing carries vanes. Within the inner casing a rotor is housed that carries blades.
Blades and vanes must withstand very heavy operating conditions, thus they need periodical checking and controls.
Different methods are known to access the vanes and blades.
For example the upper part of the outer casing and inner casing can be removed and then also the rotor can be removed, in order to access the vanes at the bottom part of the inner casing.
Nevertheless, removing the rotor is very time consuming, thus it is preferably avoided.
In addition it is known to remove the upper part of the outer and inner casing, replace the upper part of the inner casing with a ring sector and then providing rollers between the bottom part of the outer and inner casing.
This way the bottom part of the inner casing can be rotated on the rollers, bringing it at the top of the machine, such that it can be removed without removing the rotor.
The publication GB 1211313 discloses the implementation of a roller support structure for rotatably supporting the inner casing within the lower half of the outer casing. Through at least one aperture in the lower half of the outer casing the roller structure is insertable into the clearance between outer and inner casing.
The published application WO 2006103152 discloses diverse embodiments for replacing structural components of an axial turbomachine based on roller structures in the lower half of the outer casing. According to one embodiment in a first step the upper half of the inner casing is removed, in a second step the removed component is replaced by an auxiliary half ring, in a third step this auxiliary half ring is connected to the lower casing half and finally rotating the lower half and the auxiliary ring around the rotational axis until the lower half reaches an accessible area.
Even though these methods have some advantages, nevertheless damages between the adjacent parts of the inner and outer casing have been found. Tilting and axial shift of the inner casing relative to the outer casing leads to reduced clearance between the outer and the inner casing with the consequence of damages, such as fretting during roll out. To avoid this disadvantage, it is important to exactly define the axial position of the center of mass and to locate the roller support structure exactly at this axial position. But due to casting tolerances and other influences the center of mass may have shifted with the consequence of a risk of serious damages during roll out of the casing.
Both publications are silent how to avoid this disadvantage.
A further development of the solutions, as specified in the above-cited documents, discloses WO 2008012195. To maintain an exact alignment of the inner casing during its 180°-rotation around the machine axis the arrangement of two roller groups at different axial positions is proposed. Instead of one roller support at the axial position of the center of mass two roller groups, disposed at a distance to each other and on both sides of the center of mass, are applied. Though this solution avoids tilting of the casing the problem of avoiding axial shift is not solved.